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Acid can release electricity from metal.  What is an easy-to-find source of acid?   Tomatoes!

What kind of tomato is best?

Well, green tomatoes are more acidic then red, ripe ones.  So, no need to waste a good eating tomato.  If you grow your own tomatoes, you could pick one early, while still green, and use the acid from that in the battery.

Update on Tomato Battery – 4 Days Later

The video below shows how the Zinc plating was stripped off the metal by the tomato acids.  Also shown is how the LED still lights up, although with less intensity, four days after the tomato battery was constructed.

Video above shows how the AAA battery was charged from the tomato battery.  The AAA battery went up from 1318mV to 1330mV.  While not much, it shows that this small scale experiment could have a practical use.  The energy transferred from the tomato battery to the AAA battery could be used in any application that uses AAA batteries, like wireless headphones or an LED flashlight.

Is the AAA battery being overcharged?

The battery label says it is 1.2V.  If the tomato battery is charging it up to 1.33V, is that overcharging it?  My short answer is “no”.  See the video below for an explanation.

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About a week after the tomato battery was constructed, mold was growing on the tomato mix and power production dropped to nearly nothing so I disbanded this experiment and cleaned up my mess.

What kind of acid is in tomatoes?

Tomatoes have a pH of about 4.1.  The lower the pH, the stronger the acid.  To give some perspective, here are the pH values of some other acids.

0.0 Hydrochloric acid

1.0 Gastric acid (in your tummy)

2.2 lemon juice

2.4 vinegar

4.1 tomatoes

5.8 potatoes  source

6.0 urine (Yeah, your urine can be acidic and would work as a weak battery electrolyte!  It is usually more acidic when you first wake up in the morning and turns more alkaline as the day progresses.  The more meat you eat, the more acidic your urine will be.  Vegetarians can have alkaline urine.)

7.0 water (considered to be neutral and not an acid)

From my reading, it appears there are many kinds of acid present in tomatoes:

• ascorbic acid
• chlorogenic acid source
• folic acid source
• p-coumaric acid
• chlorogenic acid source
• salicylic acid source
• malic acid source
• caffeic acid source

What is the chemical reaction in the tomato battery?

In the video above, the tomato battery is initially made with:

• zinc
• copper
• acid

The Zinc is the negative terminal, the anode.  The acid from the tomato liberates electrons from the Zinc.

Zn(s)  –> Zn2+(aq) + 2 e-

Solid Zinc is dissolved into the electrolyte (the water / tomato acid mixture), turning it into a Zinc ion while releasing a couple electrons.

Copper is the positive terminal, the cathode.  The Copper atoms attract electrons more than do the Zinc (Zn) atoms.  The electrons that came off the Zinc gets sucked into the Copper terminal.

An acid’s active ingredient is positively-charged Hydrogen. The acid releases Hydrogen ions (H+), which are reduced to hydrogen gas (H2), which bubbles around the electrodes.  When I was recording the video above, I did witness bubbles forming, especially around the zinc plates.  The more bubbles, the less power the battery made.  When I lifted the plates out of the electrolyte, the bubbled vanished into thin air, literally!  After returning the plates into the electrolyte, the battery had more power without the Hydrogen bubbles covering the zinc.  By the way, if you could capture this Hydrogen gas, you could generate power from it, too!  But back to the battery…

As the current flows, more Zinc atoms go into the tomato electrolyte.  The Zinc is gradually being eaten away.

What is the chemical reaction when salt is added to the electrolyte?

A tomato is actually not necessary, nor is any acid.   Any pair of metals separated by any liquid which conducts electricity will behave as a battery.  The liquid cannot be pure water, though, and the metals must be different.

Most combinations of metals and liquids do not produce much electrical power to be practical, though.  The chemical reactions are usually not strong and soon stop producing an electric current. The first battery was invented in 1800.  In early batteries, someone often had to scrub the metal plates to remove hydrogen bubbles, oxides, and other foreign material to keep the battery working.

Salt water can make a battery, which is called an “air battery”.  However, an acid generates more power, a higher voltage and higher current, because it liberates the electrons from both the anode and the cathode.  This means both metals, in this case the copper and the zinc, both get consumed.  In an air battery (or salt water battery), only one of the metals is consumed, resulting in less power generated but also with less materials consumed.

In the acid battery, the acid and both metals are used up.  In the salt water battery, only one of the metals gets used up and the salt is never consumed.

Related post:  Make Your Own Air Battery